A typical computing power supply for laptops and related devices such as tablets uses a combination of Lithium Ion (Li-Ion) batteries, usually arranged in groups of two cells in series that produces a maximum voltage of approximately 10V. Such a relatively high power supply voltage is unsuitable for modern integrated circuits so these devices conventionally include a buck converter to regulate the battery power supply voltage from the series-connected batteries to an internal power supply voltage such as 1V for powering the integrated circuits within the devices.
A single stage multi-phase buck converter would require high voltage components to step down from such a relatively high battery power supply voltage to the relatively low internal power supply voltage. The use of such high voltage components demands substantial die space to achieve suitable drain-to-source resistance and also leads to higher gate drive losses and voltage-current overlap switching losses for the power switches. Thus, single stage multi-phase buck converters are not very efficient in applications in which the output voltage is substantially stepped down from, for example, around 10V to 1V.
To improve the efficiency and increase density, two-stage multi-phase DC/DC power converters have been developed in which a first stage multi-phase buck converter drives a second stage multi-phase buck converter with an intermediate voltage. The second stage multi-phase buck converter regulates the output voltage using the intermediate voltage as an input power supply. The second stage may use high-speed core transistors since the intermediate voltage is reduced as compared to the relatively-high battery voltage. Although such two stage voltage regulators have desirable efficiency and density in theory, their conventional implementation suffers from a number of problems. For example, efficiency is improved if the switching speed of the first stage multi-phase buck converter is considerably slower (e.g., 1%) than the switching speed of the second stage multi-phase buck converter. The relatively low speed of the first stage buck converter prevents it from adequately responding to sudden load increases or decreases.
Accordingly, there is a need in the art for multi-phase buck converters with improved response speeds.